commandserver: handle backlog before exiting

Previously, when a chg server is exiting, it does not handle connected
clients so clients may get ECONNRESET and crash:

  1. client connect() # success
  2. server shouldexit = True and exit
  3. client recv() # ECONNRESET

d7875bfbfccb makes this race condition easier to reproduce if a lot of short
chg commands are started in parallel.

This patch fixes the above issue by unlinking the socket path to stop
queuing new connections and processing all pending connections before exit.

mercurial/commandserver.py

@@ -477,11 +477,23 b' class unixforkingservice(object):'
             self._cleanup()
 
     def _mainloop(self):
+        exiting = False
         h = self._servicehandler
-        while not h.shouldexit():
+        while True:
+            if not exiting and h.shouldexit():
+                # clients can no longer connect() to the domain socket, so
+                # we stop queuing new requests.
+                # for requests that are queued (connect()-ed, but haven't been
+                # accept()-ed), handle them before exit. otherwise, clients
+                # waiting for recv() will receive ECONNRESET.
+                self._unlinksocket()
+                exiting = True
             try:
                 ready = select.select([self._sock], [], [], h.pollinterval)[0]
                 if not ready:
+                    # only exit if we completed all queued requests
+                    if exiting:
+                        break
                     continue
                 conn, _addr = self._sock.accept()
             except (select.error, socket.error) as inst: